The invention relates to a magnetic resonance imaging apparatus (MRI apparatus) comprising a gradient coil assembly for generating gradient magnetic fields in an imaging volume, the gradient coil assembly comprising at least three gradient coils for generating three different gradient magnetic fields.
Such an MRI apparatus is known in general and is widely used. In such an apparatus it is necessary to superpose strong, rapidly changing gradient magnetic fields on a very homogeneous static magnetic field. These gradient magnetic fields spatially define the imaging volume and are produced by coils carrying precisely controlled current pulses. Because of the so-called skin effect, the currents do not always flow along the intended paths in an x-, y- or z-gradient coil during or immediately after activation. Moreover, during or immediately after activation eddy currents could be induced in the other coils, in an RF screen or in other parts of the apparatus. Such effects give rise to cause time-dependent fields which cause the delay of the field to be a function of the position in space. This results in an integral field error with time as its dimension which can be written as a series of Legendre coefficients, each having one or more time constants. Consequently, artifacts may occur in certain sequences used by the MRI apparatus.
A known solution consists in the use of litz wires. The manufacturing process utilizing litz wire is expensive. The placement of the litz wire is not very precise, resulting in an unpredictable eddy current behavior in the magnet, which in its turn leads to in image quality problems. Another known solution consists in the use of narrow conductors. This is more expensive than wide conductors. It also means more dissipation and requires higher voltages.
An arrangement to minimize eddy currents in MR images is known from U.S. Pat. No. 5,555,251. In this arrangement gradient coils are positioned in the face of a pole piece, and thin disc shaped or ring shaped ferromagnetic parts of laminated layer cuts advantageously from transformer sheet material are attached to the face of the pole piece. To reduce eddy currents in these layers, narrow, radially oriented slots are cut in these layers before lamination. These layers are placed between the pole piece of the magnet and the gradient coil. Thus eddy currents in these parts are reduced only outside the gradient coil.
It is, therefore, an object of the invention to provide means for compensating self-induced eddy currents in the gradient coil assembly of an MRI apparatus as mentioned above.
This object is achieved by an MRI apparatus as claimed in claim 1 wherein a conductive element is provided in close proximity to at least one of the gradient coils in order to compensate self-induced eddy currents in the gradient coil assembly. The invention is based on the general idea to introduce pieces of a conductive material into the MRI apparatus so that the undesirable high-order behaviour can be suppressed and the nature of the short term self-eddy field becomes similar to that of the gradient coils. It has been recognized that self-induced eddy currents can best be compensated by locating the conductive element in close proximity to the gradient coil whose self-induced eddy current is to be compensated. In general, only one or several certain specific coils or all gradient coils can be provided with a conductive element which may all be identical or adapted to the respective gradient coil.
Preferred embodiments of the invention are disclosed in the dependent claims.
In preferred embodiments of the invention the conductive element is provided inside the at least one gradient coil or between an inner gradient coil element and an outer gradient coil element of the at least one gradient coil. The location of the conductive element may be a fixed part of the gradient coil in which case no apparatus-specific adjustment is required. The conductive element may also be provided in a different location within the apparatus. For example it may be integrated with the RF shield. The slitting of the RF shield can be adapted thereto, meaning that it is not designed for minimum short-term eddy currents but for the appropriate short-term eddy currents.
Furthermore, according to another aspect of the invention the conductive element comprises an active or passive coil loop which can be short-circuited in itself or can be connected to a separate loop amplifier. In both aspects the dimensions, shape and position of a short-circuited loop determine what field profile is corrected. The wire thickness determines the time constant when a short-circuited loop is used. Small current loops of this kind in principle induce only a time delay. Sometimes, however, acceleration is desirable. Positive currents can also be realized in the loop near the imaging region by connecting such a loop to a loop in the outer region. As a rule, the above short-term behaviour is determined to a high degree by the design of the conductive element.
In another embodiment of the invention the loop is driven by a signal taken from the at least one gradient coil using a transformer. Such a transformer could be made with parts of the gradient coil, for example by putting a pick-up, loop at the end of the gradient coil or on the outer side.
In another preferred embodiment of the invention the conductive element comprises a conductive pad, in particular a conductive foil or a conductive plate. Such a conductive foil may be a copper foil glued to the inner side of the gradient coil. Varying the dimension and/or slitting can restrict its effect to mainly one specific field shape and mainly one specific coil. Variation of the thickness influences the time constant and hence the special time delay. It is also possible to provide small metal plates, for example having a diameter of about 30 cm. Such conductive elements can be used for the x-, for the y- as well as for the z-gradient coil in general. The conductive pads or plates do not compensate as well as coils, that is, they do not have exactly the right field profile and interact with other coils that are switched, but this might be acceptable in many cases. The solution with conductive pads or plates is cheaper and easier to realize.